Please note that even if you manage to set it up to ask password at boot, it would not give any meaningful protection. Anybody who has access to the Deck could easily reset / change your password to anything they want and the use that password to log in.
If you want more security, you would need to encrypt the data on your SSD so it is not readable or modifiable without the right password. Setting that up is hard, you would need some knowledge about PC / Linux boot process and probably you would have issues how to enter the password before the system can show you the on-screen keyboard, etc., but it can be done.
Please note that even if you manage to set it up to ask password at boot, it would not give any meaningful protection. Anybody who has access to the Deck could easily reset / change your password to anything they want and the use that password to log in.
If you want more security, you would need to encrypt the data on your SSD so it is not readable or modifiable without the right password. Setting that up is hard, you would need some knowledge about PC / Linux boot process and probably you would have issues how to enter the password before the system can show you the on-screen keyboard, etc., but it can be done.
https://steamcommunity.com/app/1675200/discussions/0/3052863178829474785/